Question: There were $20$ cars in the parking lot. $11$ of the cars were red, and the rest were green. Complete the number sentences to find how many cars were green. $20$ cars total $- \,11$ red cars $= $
Solution: $20$ can be separated into two parts: ${11}$ and $?$ We can use subtraction to find the missing part: $20 - {11} = 9$ We can also think about how much more can be added to ${11}$ to get $20$ : ${11} + 9 = 20$ The answer $ 20 - {11} = 9 $ ${11} + 9 = 20$